1. Field of the Invention
The present invention relates to implantable medical devices, which deliver energy to stimulate tissue for the purposes of providing therapy to the tissue of an animal, and more particularly to an radio frequency amplifier for use in such a medical device.
2. Description of the Related Art
A remedy for a patient with a physiological ailment is to implant an electrical stimulation device that provides provide therapy to the patient. An electrical stimulation device is a small electronic apparatus that stimulates an organ or part of an organ with electrical pulses. It includes a pulse generator, implanted in the patient, and from which electrical leads extend to electrodes placed adjacent to specific regions of the organ.
An improved apparatus for physiological stimulation of a tissue includes a wireless radio frequency (RF) receiver implanted as part of a transvascular platform that comprises at least one stent-electrode that is connected to the wireless RF receiver and an electronic capsule containing a stimulation circuitry. The stimulation circuitry receives the radio frequency signal and, from the energy of that signal, derives an electrical voltage for powering the implanted device. The electrical voltage is applied in the form of suitable waveforms to the electrodes, thereby stimulating the tissue of the organ.
The radio frequency (RF) signal generation is a significant part of the electrical stimulation apparatus and it usually involves the use of an RF amplifier. The RF amplifier of choice typically has been a Class-A or Class-AB amplifier in those cases where linearity is of utmost concern. The class of an analog amplifier defines what proportion of the input signal cycle is used to actually switch on the amplifying device. A Class-A amplifier is switched on 100% of the time. A Class-AB amplifier uses a signal cycle that is greater than 50%, but less than 100% to switch on the amplifying device. Unfortunately, these amplifiers are not very efficient and dissipate a significant amount of energy. The efficiency of a power amplifier is defined as the ratio of output power and input power expressed as a percentage.
Recently, a different kind of amplifier, known as a switching amplifier, has been developed. A particularly useful switching amplifier is called a Class-E amplifier. Switching amplifiers have relatively high power efficiency due to the fact that perfect switching operation does not dissipate power. An ideal switch has zero impedance when closed and infinite impedance when open, implying that there is zero voltage across the switch when it conducts current (on state) and zero a non-zero voltage across it in the non-conductive state (off state). Consequently, the product of voltage and current (power loss) is zero at any time. Therefore, a Class-E amplifier has a theoretical efficiency of 100%, assuming ideal switching.
From a theoretical standpoint, a Class-E amplifier can provide very efficient RF amplification. However, in practice, Class-E amplifiers do not achieve anywhere close to the theoretical limits. Some embodiments of the prior art techniques use a relaxation oscillator to drive the amplifier. With this technique, it is impossible to control the range of the power depending on the need. In other embodiments, a regulator is used to control the power feed. In this case, heat is generated in the control system itself and the amplifier's efficiency is subsequently lowered. Therefore, there is a need to improve the performance of practical Class-E RF power amplifiers based on the fundamental understanding of the loss generation processes. An optimal design can make the heat dissipation so low such that heat-sink are not required.